I have the 21s and just replaced the Pirellis with Michelins at 39k miles. I kept the 'best' Pirelli and got a used OEM 21 wheel for ~$200, and that's my spare now. Assuming you can find a 20" wheel for about the same price, you'd still come out ahead getting both a 20 spare and a 21 spare and...
I once parked my R1T and walked past an old lady who practically spat out "blah blah Tesla blah blah" under her breath to her traveling companion who grabbed onto her. For a moment I was afraid she was going to hit me with her walker. I don't mind people who don't like trucks or EVs, but I draw...